As is generally known, the spent fuel elements of nuclear reactors and compositions or mixtures thereof contain residual fissionable components that may be further utilized as a nuclear fuel. Besides these fissionable components, there are also present fuel elements and mixtures thereof, which contain construction metallic components having a low coefficient of absorption for thermal neutrons. Such construction metallic components are zirconium alloys containing tin, niobium, titanium, and other elements. The fissionable components are, in general, hard compounds of uranium and plutonium produced by ceramic methods. The fuel component is usually enclosed in thin-wall tubes made of zirconium alloys forming elements that are connected into or associated with larger assemblies enclosed in a casette tube. In addition, the assemblies contain fastening construction parts from non-fissionable metals and alloys. The single parts of the fuel assembly, when removed from the reactor, are deformed and are highly radioactive.
When nuclear fuels are reprocessed, difficulties arise during separation of the fissionable and non-fissionable components required for biological protection. The difficulty in separating fissionable and non-fissionable elements is enhanced by the different mechanical and chemical properties of these components. Uranium and plutonium compounds are extremely hard and brittle; zirconium alloys, on the other hand, are firm and tough. Many methods for improving the separation of these components have been proposed involving the use of mechanical, thermal and mechanical and chemical procedures. For example, G. Manile and G. Matchret disclosed such a procedure in U.S. Pat. No. 3,664,104, wherein the spent elements are cut into short fragments which are then mechanically processed in a ball mill. In this process, ceramic particles of the fissionable components are broken, then mechanically separated and thereafter are chemically treated. A disadvantage accompanying this method is the difficulty in cutting the metal-ceramic composite. Further, the steps in this breakdown into separate elements are lengthy and because of the need to exercise extreme precautions in the protection against nuclear radiation, this procedure is very complicated and expensive. In addition, the deformed assemblies, especially when they are very large, are difficult to handle.
Another method for the reprocessing of fuel elements in stainless steel cladding is described by R. E. Strong in British Specification No. 1,274,357/1972/which consists in melting off the metallic component of the element by means of induction heating. Chemical and electrochemical methods of cladding separation are disclosed by P. Ballot in French Pat. No. 2,081,176/1971.
The disadvantage associated with the induction heating method is the fact that this procedure is not technically and economically suitable for the separation of zirconium cladding from the ceramic component. In addition, the high melting temperature and the extremely high chemical reactivity of zirconium towards oxygen, carbon, and nitrogen leading to the simultaneous formation of high-melting compounds make the separation of both components virtually impossible. Further, heating the nuclear components to high temperatures increases the danger of contamination by volatile components.
For this reason, it seemed reasonable and advantageous to find a method for the separation of uranium, plutonium, and their compounds that would not have the drawbacks of the known methods.